In a wireless communication system, the channel status to a serving cell is generally changed from time to time depending on the movement or communication environment of a User Equipment (UE), and if the channel status of the serving cell is not good, the UE may declare Radio Link Failure (RLF) to perform Radio Resource Control (RRC) connection reconfiguration, or may detect (or search for) a target cell for handover among the neighboring cells to perform a handover procedure.
First, a handover procedure in a Long Term Evolution (LTE) system that is an example of the wireless communication system will be described.
FIG. 1 illustrates a handover procedure in an LTE system according to the related art. Herein, the LTE system may include a 3rd Generation Partnership Project (3GPP) LTE-Advanced (LTE-A) system.
Referring to FIG. 1, in operations 101 to 107, if a UE 110 detects a handover event by measuring the strength (e.g., Reference Signal Received Power (RSRP)) of a downlink signal, the UE 110 may report the result to a serving evolved Node B (eNB) 130 of a serving cell, using a measurement report message. In operations 109 to 119, the serving eNB 130 may determine whether to perform handover, and request handover from a target eNB 150 of a target cell to which the UE 110 is to be handed over. The target eNB 150 may determine whether to admit handover of the UE 110 by performing admission control. If handover of the UE 110 is admitted, the serving eNB 130 may transmit the information used to perform handover to the UE 110, using a handover command message. In operations 121 to 127, the UE 110 may perform a handover procedure to the target eNB 150, using the information included in the handover command message, and the UE 110 may terminate the handover procedure after sending a handover confirm message to the target eNB 150. Referring to FIG. 1, reference numerals 11, 13 and 15 represent individual intervals determined by dividing the handover procedure. Reference numeral 11 represents a Time To Trigger (TTT) interval in which handover is triggered, and the TTT interval may be maintained while a handover timer TTT is running.
Generally, in the wireless communication system, handover may occur if the signal strength of the target eNB is greater than the signal strength of the serving eNB, meaning that the UE is located at the edge of the serving cell or a channel gain between the UE and the serving eNB is low. Therefore, the handover that occurs because the signal strength of the target eNB is higher than the signal strength of the serving eNB, and the RLF that occurs because the channel gain between the UE and the serving eNB is low, are highly likely to occur together. In the LTE system, the handover and the RLF are defined through the conditions in the following Table 1.
TABLE 1Condition maintainingConditionstime (timer)HandoverRSRPtarget > RSRPserving + ΔTTT (time-to-trigger)RLF‘BLERPDCCH < BLERthreshold’ has T310occurred N310 times (out-of-sync indication is received N310 times)
In Table 1, “N310” is set to a predetermined number of times. As shown in Table 1, if the handover and RLF conditions are maintained for a specified time (e.g., timer TTT, and T310), the handover and RLF may be declared and an operation related thereto may be performed. If the handover and RLF conditions haven't been maintained for the specified time even though the handover and RLF conditions are satisfied, the conditions that have already occurred may be invalid.
Basically, the handover and the RLF may be the processes that are operated independently. Therefore, if the RLF is declared while the handover procedure is performed, the UE may perform RRC connection reconfiguration regardless of the remaining handover procedure. The handover and the RRC connection reconfiguration may cause service disconnection while performing the following operations, respectively.
TABLE 2OperationsHandoverDownlink synchronization → uplink synchronization →resource allocationRRC Cell search → downlink synchronization → systemconnectioninformation acquisition → uplink synchronization →reconfigurationresource allocation
As shown in Table 2, the RRC connection reconfiguration, compared with the handover, uses additional time for cell search and system information acquisition. Generally, the cell search and the system information acquisition are operations using a longer time than downlink synchronization, uplink synchronization, and resource allocation. Therefore, it can be noted that when the UE desires to perform communication with a new eNB by performing handover or RRC connection reconfiguration by RLF, the handover is advantageous over the RRC connection reconfiguration in terms of the service disconnection time.
In a heterogeneous network including various types of cells, the frequency of co-occurrence of the handover and the RLF has increased due to the introduction of small cells. The reason is as follows. First, the small cell has a small cell coverage area. If the small cells are installed at a high density, the UE may detect multiple small cells and may be subject to interference from them. Therefore, the number of handover areas may increase, and the number of areas where the RLF condition is satisfied due to the inter-cell interference may also increase. To cope with this, an operation of considering the handover and the RLF at the same time has been proposed recently. Details thereof are as follows.
FIG. 2 illustrates a scheme for early termination of an RLF timer in an LTE system according to the related art.
Referring to FIG. 2, while an RLF timer T310 is running, if a handover timer TTT is started, maintained and terminated (or expired), the UE may terminate the RLF timer early upon expiration of the handover timer TTT as shown by reference numeral 201, without waiting for the remaining time of the RLF timer until the end, and then perform RRC connection reconfiguration.
As described above, the RLF may occur when the channel gain between the UE and the serving eNB is low, and the RLF may be determined by a Block Error Rate (BLER) of a Physical Downlink Control Channel (PDCCH). Therefore, referring to FIG. 2, the early termination technique for an RLF timer is to terminate the RLF timer early to reduce the waiting time that is unnecessary in performing RRC connection reconfiguration, determining that the UE is highly unlikely to successfully receive a handover command message from the serving eNB since the RLF timer of the UE has been started.
However, the reason why the UE uses (or runs) the RLF timer without directly declaring the RLF is that the channel gain might be likely to get better again during the RLF timer. For this reason, in the early termination technique for an RLF timer according to the related art, the UE may not monitor the situation where the channel gain of the serving cell gets better again. In addition, since the UE performs RRC connection reconfiguration at all times after terminating the RLF timer early, the RLF may undergo a more complex procedure, compared with the RLF which may be automatically recovered.
In addition, the fact that the RLF timer is running means that the channel gain between the UE and the serving eNB is very low on average, but the instantaneous channel gain might be likely to be high by fast fading. If this possibility is taken into consideration, even though the RLF timer is running, the UE is likely to successfully receive a handover command message and successfully perform handover. Therefore, quickly performing RRC connection reconfiguration at all times like in the early termination technique of the related art for an RLF timer T310 may not be considered the best solution.
Therefore, in order to address the disadvantages of the early termination technique for an RLF timer, a new timer called a T312 timer has been introduced. For convenience, the T312 timer will be referred to herein as a waiting timer. The technique for utilizing a waiting timer T312 may operate as follows.
FIG. 3 illustrates an operation of a proposed waiting timer T312 in an LTE system according to the related art.
Referring to FIG. 3, while the RLF timer T310 is running, if the handover timer TTT is started, and then terminated at time 301, the UE may perform a handover procedure during the waiting timer T312, and if the waiting timer T312 is terminated at time 303, the UE may terminate the RLF timer early without waiting for the remaining time of the RLF timer until the end, and then perform RRC connection reconfiguration. The technique for a waiting timer T312 in FIG. 3 has been provided by partially improving the early termination technique for an RLF timer, which has been described in FIG. 2.
The early termination technique for an RLF timer in FIG. 2 has not considered the possibility that the channel gain will be recovered again, and the possibility that the UE will successfully receive a handover command message since the instantaneous channel gain is high even though the overage channel gain is low. However, by introducing the waiting timer T312 as shown in FIG. 3, the UE may monitor whether the channel gain is recovered or the handover is successfully performed during the waiting timer T312, even after the handover timer TTT is terminated. In addition, if the waiting timer T312 is terminated, the UE may terminate the RLF timer early, and then perform RRC connection reconfiguration, thus making it possible to prevent the increase in the service disconnection time of the UE.
However, as to the technique for a waiting timer T312, there are no specific proposed measures for determining by which scheme the UE will operate the waiting timer T312 in various situations which may occur in the network. Therefore, the measures to control the waiting timer T312 more efficiently are desired.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.